3.8.1 解答

Tue, 11 Aug 2026 11:30:00 +0800
译文 · 基于 Comprehensive Rust
原文链接: https://google.github.io/comprehensive-rust/lifetimes/solution.html
3.8.1 解答

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// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0
/// 线上所见的线类型（wire type）。
enum WireType {
 /// Varint 线类型表示值是单个 VARINT。
 Varint,
 // I64 线类型表示值恰好是 8 个小端序字节，
 // 包含 64 位有符号整数或 double。
 //I64, -- 本练习不需要
 /// Len 线类型表示值是一个以 VARINT 表示的长度，后跟恰好该数量的字节。
 Len,
 // I32 线类型表示值恰好是 4 个小端序字节，
 // 包含 32 位有符号整数或 float。
 //I32, -- 本练习不需要
}

#[derive(Debug)]
/// 字段的值，按线类型区分。
enum FieldValue<'a> {
 Varint(u64),
 //I64(i64), -- 本练习不需要
 Len(&'a [u8]),
 //I32(i32), -- 本练习不需要
}

#[derive(Debug)]
/// 一个字段，包含字段编号及其值。
struct Field<'a> {
 field_num: u64,
 value: FieldValue<'a>,
}

trait ProtoMessage<'a>: Default {
 fn add_field(&mut self, field: Field<'a>);
}

impl From<u64> for WireType {
 fn from(value: u64) -> Self {
 match value {
 0 => WireType::Varint,
 //1 => WireType::I64, -- 本练习不需要
 2 => WireType::Len,
 //5 => WireType::I32, -- 本练习不需要
 _ => panic!("Invalid wire type: {value}"),
 }
 }
}

impl<'a> FieldValue<'a> {
 fn as_str(&self) -> &'a str {
 let FieldValue::Len(data) = self else {
 panic!("Expected string to be a `Len` field");
 };
 std::str::from_utf8(data).expect("Invalid string")
 }

 fn as_bytes(&self) -> &'a [u8] {
 let FieldValue::Len(data) = self else {
 panic!("Expected bytes to be a `Len` field");
 };
 data
 }

 fn as_u64(&self) -> u64 {
 let FieldValue::Varint(value) = self else {
 panic!("Expected `u64` to be a `Varint` field");
 };
 *value
 }
}

/// 解析 VARINT，返回解析出的值与剩余字节。
fn parse_varint(data: &[u8]) -> (u64, &[u8]) {
 for i in 0..7 {
 let Some(b) = data.get(i) else {
 panic!("Not enough bytes for varint");
 };
 if b & 0x80 == 0 {
 // 这是 VARINT 的最后一个字节，因此将其转换为
 // u64 并返回。
 let mut value = 0u64;
 for b in data[..=i].iter().rev() {
 value = (value << 7) | (b & 0x7f) as u64;
 }
 return (value, &data[i + 1..]);
 }
 }

 // 超过 7 个字节无效。
 panic!("Too many bytes for varint");
}

/// 将标签转换为字段编号与 WireType。
fn unpack_tag(tag: u64) -> (u64, WireType) {
 let field_num = tag >> 3;
 let wire_type = WireType::from(tag & 0x7);
 (field_num, wire_type)
}

/// 解析一个字段，返回剩余字节
fn parse_field(data: &[u8]) -> (Field<'_>, &[u8]) {
 let (tag, remainder) = parse_varint(data);
 let (field_num, wire_type) = unpack_tag(tag);
 let (fieldvalue, remainder) = match wire_type {
 WireType::Varint => {
 let (value, remainder) = parse_varint(remainder);
 (FieldValue::Varint(value), remainder)
 }
 WireType::Len => {
 let (len, remainder) = parse_varint(remainder);
 let len = len as usize; // 为简单起见做类型转换
 let (value, remainder) = remainder.split_at(len);
 (FieldValue::Len(value), remainder)
 }
 };
 (Field { field_num, value: fieldvalue }, remainder)
}

/// 在给定数据中解析消息，对消息中的每个字段调用 `T::add_field`。
///
/// 整个输入都会被消费。
fn parse_message<'a, T: ProtoMessage<'a>>(mut data: &'a [u8]) -> T {
 let mut result = T::default();
 while !data.is_empty() {
 let parsed = parse_field(data);
 result.add_field(parsed.0);
 data = parsed.1;
 }
 result
}

#[derive(PartialEq)]
#[derive(Debug, Default)]
struct PhoneNumber<'a> {
 number: &'a str,
 type_: &'a str,
}

#[derive(PartialEq)]
#[derive(Debug, Default)]
struct Person<'a> {
 name: &'a str,
 id: u64,
 phone: Vec<PhoneNumber<'a>>,
}

impl<'a> ProtoMessage<'a> for Person<'a> {
 fn add_field(&mut self, field: Field<'a>) {
 match field.field_num {
 1 => self.name = field.value.as_str(),
 2 => self.id = field.value.as_u64(),
 3 => self.phone.push(parse_message(field.value.as_bytes())),
 _ => {} // 跳过其他一切
 }
 }
}

impl<'a> ProtoMessage<'a> for PhoneNumber<'a> {
 fn add_field(&mut self, field: Field<'a>) {
 match field.field_num {
 1 => self.number = field.value.as_str(),
 2 => self.type_ = field.value.as_str(),
 _ => {} // 跳过其他一切
 }
 }
}

#[test]
fn test_id() {
 let person_id: Person = parse_message(&[0x10, 0x2a]);
 assert_eq!(person_id, Person { name: "", id: 42, phone: vec![] });
}

#[test]
fn test_name() {
 let person_name: Person = parse_message(&[
 0x0a, 0x0e, 0x62, 0x65, 0x61, 0x75, 0x74, 0x69, 0x66, 0x75, 0x6c, 0x20,
 0x6e, 0x61, 0x6d, 0x65,
 ]);
 assert_eq!(person_name, Person { name: "beautiful name", id: 0, phone: vec![] });
}

#[test]
fn test_just_person() {
 let person_name_id: Person =
 parse_message(&[0x0a, 0x04, 0x45, 0x76, 0x61, 0x6e, 0x10, 0x16]);
 assert_eq!(person_name_id, Person { name: "Evan", id: 22, phone: vec![] });
}

#[test]
fn test_phone() {
 let phone: Person = parse_message(&[
 0x0a, 0x00, 0x10, 0x00, 0x1a, 0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x33,
 0x34, 0x2d, 0x37, 0x37, 0x37, 0x2d, 0x39, 0x30, 0x39, 0x30, 0x12, 0x04,
 0x68, 0x6f, 0x6d, 0x65,
 ]);
 assert_eq!(
 phone,
 Person {
 name: "",
 id: 0,
 phone: vec![PhoneNumber { number: "+1234-777-9090", type_: "home" },],
 }
 );
}

// 把以上全部合到一次解析中。
#[test]
fn test_full_person() {
 let person: Person = parse_message(&[
 0x0a, 0x07, 0x6d, 0x61, 0x78, 0x77, 0x65, 0x6c, 0x6c, 0x10, 0x2a, 0x1a,
 0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x30, 0x32, 0x2d, 0x35, 0x35, 0x35,
 0x2d, 0x31, 0x32, 0x31, 0x32, 0x12, 0x04, 0x68, 0x6f, 0x6d, 0x65, 0x1a,
 0x18, 0x0a, 0x0e, 0x2b, 0x31, 0x38, 0x30, 0x30, 0x2d, 0x38, 0x36, 0x37,
 0x2d, 0x35, 0x33, 0x30, 0x38, 0x12, 0x06, 0x6d, 0x6f, 0x62, 0x69, 0x6c,
 0x65,
 ]);
 assert_eq!(
 person,
 Person {
 name: "maxwell",
 id: 42,
 phone: vec![
 PhoneNumber { number: "+1202-555-1212", type_: "home" },
 PhoneNumber { number: "+1800-867-5308", type_: "mobile" },
 ]
 }
 );
}
3.8 练习：Protobuf 解析 on 编程那些事儿

3.8.1 解答

3.8.1 解答
